In modern semiconductor device applications, millions of individual devices may be packed onto a single small area of a semiconductor substrate. Many of these devices may need to be electrically isolated from one another. One method of accomplishing such isolation is to form a trench isolation region between adjacent devices.
Various insulative materials have been formed within trenches for trench isolation. For instance, thermally grown silicon dioxide may be formed to line the trenches, silicon nitride formed over the thermally-grown silicon dioxide, and a thick filler of silicon dioxide formed over the silicon nitride. The thick filler of silicon dioxide may be formed by chemical vapor deposition (CVD) or high-density plasma chemical vapor deposition (HDP-CVD). A high density plasma is a plasma having a density of greater than 1010 ions/cm3.
The utilization of CVD and/or HDP-CVD to form oxide may lead to incomplete filling of at least some of the trenches, which may create non-uniformity of isolation across a semiconductor substrate.
Another method which may be utilized to form thick layers of oxide in addition to, or alternatively to, CVD or HDP-CVD is oxidation of a spin-on material. For instance, polysilazane film may be formed across a substrate by a spin-on process, and then converted to silicon dioxide. Polysilazane has a structural formula of [SiNR1R2R3]n where R1, R2 and R3 are all hydrogen in the case of inorganic polysilazane; and are alkyl, aryl or alkoxyl organic moieties in organic polysilazane. The conversion of polysilazane to silicon dioxide may be accomplished utilizing steam at a temperature of from about 600° C. to about 1050° C.
Unfortunately, the high temperature steam oxidation attacks silicon and may result in consumption of a large amount of transistor active area real estate. One method of alleviating this problem during fabrication of dynamic random access memory (DRAM) on a silicon substrate is to form a thin layer of thermally grown silicon dioxide, followed by a thin layer of silicon nitride. The thin silicon dioxide layer and thin silicon nitride layer together form a barrier to oxidation which protects against consumption of active area silicon.
The utilization of the silicon nitride causes additional process steps which reduce throughput. Further, utilization of silicon nitride is not practical during fabrication of some types of memory. For instance, silicon nitride films may be problematic during fabrication of NAND memory due to problems of leakage and charge trapping that may occur if the silicon nitride is too near to tunnel dielectric of the memory cells utilized in the NAND memory.
High-temperature steam oxidation may further create complications during fabrication of integrated circuitry in that some devices may not tolerate the high temperature utilized for the oxidation.
It would be desirable to develop new methods of forming trench isolation which avoid one or more of the above-discussed problems.